[Greenbuilding] Water heaters/Solar

[Keith Winston]

Re: Dawn Solar: I don't think we've been unfair enough ;-)

Dawn solar has abysmal results. Look at the numbers from the SRCC, 
solar-rating.org:

They have 30 Btu/sf/day on a clear day with a delta T of 36F. That 
means, let's say it's 50F out, you don't require water any warmer than 
86F. As soon as it gets any colder, or your require hotter water, or it 
gets even mildly cloudy, their performance drops to zero. Literally, 
according to the tests.

For comparison, a decent flat plate collector might produce 1000 
Btu/sf/day in those conditions, and go on to produce 600 at a 90F delta 
T, a MUCH more useful delta T.

There's a very interesting report from HREL about RISA, Roof Integrated 
Solar Absorbers. They find:

Water Heating
• Glazed collectors are ~10-15 times more efficient (per
unit area) than metal roof RISAs
• Metal roof RISAs are ~10 times more efficient than
asphalt shingle RISAs
Pool Heating
• Unglazed collectors are ~2-3 times more efficient (per
unit area) than metal roof RISAs
• Metal roof RISAs are ~2-3 times more efficient than
asphalt shingle RISAs

Now, they go on to say:

Conclusions:
• Roof-integrated solar absorbers are able to
provide ~1 kWh/m2/day of hot water energy (~ the
same solar conversion efficiency as PV systems!)
• For both hot water and pool heating, roof integrated
solar absorbers are an aesthetic
alternative to roof-mounted solar collectors.
• Roof-integrated solar absorbers are also able to
reduce solar heat gain in cooling load-dominated
climates.

But if you do the math, I don't think that's right. I find glazed flat 
plate heat exchangers to be about 40-70% efficient, when all is said and 
done. PV is 15-20% efficient (okay, you can't really buy 20% efficient 
modules yet, but they're available commercially). The SRCC number back 
up the "10-15 times more efficient" than the RISA's, but that means 
they're more like 5% efficient, somewhat lower than the trashiest thin 
film PV you can buy (8% give or take). Plus, they didn't distinguish in 
their final comments the temperature issue: in their charts, they show a 
metal roof RISA (the better type) dropping from 368 Btu/sf/day to 53 
when the delta T goes from 35C to 50C (to be fair, these are far better 
output numbers than the SRCC data for Dawn Solar). The NREL paper 
suggests about 8% efficiency, for hot water heating in Florida. That is, 
in a warm climate, and not for space heating. Let me repeat that: 
they're talking about hot water heating and pool heating in Florida. Any 
wind, or higher delta T, and performance goes out the window. Still, 
their conclusions are much more encouraging than I am inclined to be.

http://www.nrel.gov/docs/fy01osti/30848.pdf

Note that they say pool heaters (unglazed) are 2-3 times more efficient 
than the best RISA's. Note that no one uses pool heaters for space 
heating, nor even for hot water heating in cold climates...

Finally, I don't think it's cheap, unless you're a do-it-yourselfer. 
Then it could be cheap, if you have a vented attic with easy access. If 
you do an on-top-of-sheathing install, like Dawn solar suggests last I 
looked, then you need a second set of sheathing, which has to drive the 
price to an unrealistic level... Now, I'm just thinking now of a friend 
that does metal 5V roofs in New England over typar with blown cellulose, 
no sheathing. If you could figure out how to attach your pex to your 
metal roofing directly, then you eliminate heat transfer plates, etc. 
Hmmm. Then you'd just need to figure out a monitor/control system that 
sweeps any heat out of the roof before the next breeze comes along... A 
one-wire string of temp sensors could do it. Hmmm... Still, it's glazing 
that makes the difference in cold weather ...

Keith



barbara deane-gillett wrote:
> i think that's unfair.  
>  
> dawn solar has a unique product based on the idea that because people would like solar systems to be invisible, one can sacrifice some performance and obtain an attractive functional system at a reasonable price.  they have designed the system to be integral with and installed under a metal roof by roofers. because the system has no outer glazing, it works best as a solar dhw preheater (nearly all solar dhw systems are preheaters in winter, so you don't lose much)  or a radiant floor heater.  it of course has more trouble in colder climates, but for the homeowner who wants appearance first and performance second.
>  
>  it is a reasonable alternative particularly for the owner already interested in a metal roof.  the system is fsec rated and qualifies for the credits. because the system is relatively inexpensive per sq.ft ( $20 vs $100)  one can install more sq.ft to make up for some performance loss of not having a glazing.. 
>  
> they also have a unique version which has pv glued to the metaql roof. a small side benefit is that cooling the pv with the solar dhw slightly increases the pv performance. 
>  
> they have been at it now for over five years with reasonable results.> Date: Mon, 5 Nov 2007 21:52:45 -0600> From: Unschooler at atlasok.com> To: kbeiser at centurytel.net; GREENBUILDING at LISTSERV.REPP.ORG> Subject: Re: [Greenbuilding] Water heaters/Solar> > Ken,> > If you go to the archives and search for "Dawn Solar" you'll read > several past conversations about it. They have generally been negative.> > --Leslie> > > Ken Beiser wrote:> > Sorry for the cross post but the subject of solar water heaters under metal roofs is on my mind right now.> >> >> > > > _______________________________________________> Greenbuilding email list> Environmentally-preferable design, construction, building elements> List info: http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> List email: Greenbuilding at listserv.repp.org> Managed by BuildingGreen, Inc. http://www.buildinggreen.com> publisher of Environmental Building News and GreenSpec> Hosted and archived by REPP / CREST http://www.crest.org> To get on the list:> http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> or mailto:greenbuilding-request at listserv.repp.org?subject=unsubscribe> To get off the list:> http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> or mailto:greenbuilding-request at listserv.repp.org?subject=unsubscribe
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